How excavation volume is calculated
Pit size = (L + 2 × working space) × (W + 2 × working space) × depth
Bank volume = pit size × number of pits
Loose volume = bank volume × bulking factor
Bank volume = pit size × number of pits
Loose volume = bank volume × bulking factor
Soil swells once it is dug out, because it loosens and traps air. That is why the volume in the trucks is bigger than the hole. The bulking factor depends on the soil type.
Backfill and surplus
Backfill is the space left around the footings after the structure is built: bank volume minus the volume of the structure. Whatever is not needed for backfill is surplus and has to be carted away, measured as loose volume.
Bulking factors by soil type
| Material | Bulking |
|---|---|
| Sand | 10–15% |
| Sand and gravel | 15% |
| Silty soil | 20–25% |
| Clay | 30–40% |
| Broken rock | 50–60% |
Frequently asked questions
Is excavation measured as bank or loose volume?
Most BOQs measure excavation as the bank (in-place) volume of the hole. Carting away is often priced on loose volume, so check your contract's method of measurement.
How much working space should I allow?
Typically 300–600 mm on each side of the footing for formwork and waterproofing, more in deep or unstable ground where shoring or side slopes are needed.